A printed-circuit board is a dielectric plate carrying on one or both faces a network of lines of a conductive material such as copper that are either printed on the plate or created by etching out the copper between the lines on a completely copper-plated plate. The board is drilled at the conductive lines to create holes that allow the leads of circuit elements to be soldered in place and that allow if necessary connection to be made from a line on one face of the board to a line on the opposite face. Before circuit elements are soldered to the board it is necessary to test it, both to ascertain that each of the lines is electrically continuous, and to verify that there are no unwanted connections between lines that are supposed to be electrically independent. Any error whatsoever of continuity or discontinuity makes the board unusable, a reject.
The testing apparatus has a probe assembly with a multiplicity of finger-like contacts whose tips are simultaneously brought into contact with the various lines. The contacts are arrayed such that one contact engages each end of each conductive line or the end of any extension thereof.
To accommodate different sizes of boards the support board of the probe assembly is provided with a multiplicity of densely spaced depressible supports. A tight grid of such supports is provided, with the supports independently connected to an electronic test and analyzing device, and the grid is at least as large as the largest board to be tested. Thus when the board to be tested and the array of supports are juxtaposed, there will be at least one support generally aligned with the end of every line or segment thereof as well as with any bores in the board. The contacts are rods supported on those supports which lie generally in the right position relative to the locations to be contacted. Only those contacts needed for a specific test are mounted on supports, and a mask is provided to orient the forward ends of the contact rods appropriately for the specific board being tested. The supports allow the contact rods to be depressed back into the probe assembly so that when the board is passed back or the assembly is pressed up against the board the contacts will all bear with a certain standard pressure against the points being tested, thereby making the resistance at the interface between the front end of the contact and the line being tested uniform for all the contacts.
In order to permit the contacts to cant somewhat so they can be tipped to conform to a particular pattern it is standard to form their rear ends, that is their ends turned away from the sample being tested, as mainly spherical balls and to receive them in conical or frustoconical seats on the front ends of the depressible supports. In this manner the extent of contact will be the same even if the axis of the contact rod is not coaxial with the movement axis of the depressible support.
The socket of such an arrangement therefore has a contact surface that is highly susceptible to getting dirty in use. The contact rods sit in these sockets and in fact are only retained therein by the force of gravity. Thus the sockets open upward and readily receive and dirt or the like filtering down from above through the perforated mask. Once the surface gets dirty or corrodes somewhat the contact resistance at this critical joint between the contact and its support increases greatly. This nearly open circuit at this joint can therefore cause the testing apparatus to read as open-circuited a line or connection of the circuit board and cause the board to be rejected, as only error-free boards are used.
It is to be noted that a probe assembly normally includes a large number of respective identical contact elements, whose plungers are disposed parallel to one another therewithin according to a predetermined and normally very fine grid or pattern. The plungers are spring-loaded and movable in an axial direction. The probe assembly is operatively so arranged that the plungers are vertically disposed and that the upright contact rods are carried thereby. Here the ball joint of each contact element may be held together by gravity acting on the contact rod, so that no additional measures are necessary to hold the ball joint of each contact element together. While all plungers in a probe assembly may be located in the corresponding grid, and may be present permanently, a corresponding number or a lesser number of contact rods may be located in the through-going bores of a longitudinally movable guide plate in accordance with the requirements of the respective conductive plates to be contacted. But the test contacts of a conductive plate, which are to be contacted simultaneously by the contact rods, are frequently not disposed in a pattern corresponding to that of the plunger grid. Furthermore, the number of the test contacts is frequently less than the number of the points in the grid occupied by the plungers. Consequently the ball joints make it possible to deflect or pivot the contact rods from the grid of the plungers so that they assume a somewhat inclined position, thus deviating from the vertical. For each type of conductive plate to be tested there is then prepared a guide plate having through-going bores for the contact rods, namely in correspondence with the test points to be examined or tested. Subsequently the guide plate is installed in the probe assembly, thereafter the contact rods are inserted into the bores of the guide plate, and then allowed to fall onto the nearest plunger by the action of gravity. The test point of a conductive plate is defined as a location in such a conductive plate or the like which is to be contacted by the respective contact element. Thus this may relate to a location in a conductive track, to a contact sleeve, and the like. The invention relates in particular to contact elements for probe assemblies of the aforedescribed type.